Thin film transistor and a method of forming the same

ABSTRACT

A method of forming a thin film transistor, includes: forming an active region on a first insulating layer, the active region having a channel region, at least one sub-channel region, and first regions disposed between the channel region and each sub-channel region; sequentially forming a second insulating layer and a first conductive layer over the first insulating layer; patterning the second insulating layer and the first conductive layer to form a gate insulating layer and gate electrode on a channel region of the active layer, and to form a sub-gate insulating layer and associated sub-gate electrode on each sub-channel region of the active layer; forming a mask covering at least a portion of the gate electrode, at least a portion of each sub-gate electrode, and each first region of the active region; and implanting impurities into exposed portions of the active region using the mask to form a source region on a first side of the channel region and a drain region on a second side of the channel region such that each sub-gate region is disposed between the channel region and one of the drain region and the source region.

This application is a divisional of U.S. application Ser. No. 09/882,0 filed on Jun. 18, 2001, which is a continuation application of U.S. application Ser. No. 08/859,688 filed on May 21, 1997 and now abandoned, the entire contents of each of the applications are hereby incorporated by reference nd for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 96-22713 filed in Korea on Jun. 21, 1996 under 35 U.S.C. §119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor and a method of forming the same, and more particularly, to a lightly doped drain (LDD) or offset structure of a thin film transistor providing enhanced performance by uniformly forming an LDD region or offset region.

2. Description of Related Art

To date, amorphous silicon thin film transistors constitute a main component of a conventional active matrix liquid crystal display (AMLCD). Compared to amorphous silicon, polysilicon has higher mobility, and, if used in a liquid crystal display, does not require a separate drive circuit. Instead, the drive circuit can be formed internally.

A recently developed polysilicon thin film transistor liquid crystal display includes a drive circuit and pixel array formed together on a transparent glass substrate. The thin film transistors of the drive circuit allow high frequency switching due to the use of polysilicon. However, thin film transistors used for pixel switching in a pixel array interfere with the switching of the thin film transistors in the drive circuit because the high drain current generated during an off state of the pixel switching transistors. In order to lower this off current to a predetermined level, thin film transistors incorporating an LDD structure, offset structure, and a dual gate structure have been suggested.

FIG. 1 shows a plan view of a portion of a pixel in a conventional liquid crystal display which uses a thin film transistor as a pixel switching element. FIG. 2 shows a cross-sectional view of the conventional liquid crystal display, taken along line II—II of FIG. 1.

As shown in FIG. 1, a signal line 13′ and a data line 18′ cross each other on an insulating substrate 10. The gate electrode 13 of a thin film transistor extends from the signal line 13′, and the source electrode 18 of the thin film transistor extends from the data line 18′. The drain electrode 19 of the thin film transistor, which faces the source electrode 18, is connected to a pixel electrode 21. A first storage capacitor electrode 24 crosses over data line 18′. The pixel electrode 21, formed of a transferring conductive material on the first storage capacitor electrode 24, functions as a second storage capacitor electrode.

As shown in FIG. 2, a semiconductor active layer 11 is formed on an insulating substrate 10. The semiconductor active layer 11 is made of a polysilicon thin film. On the sides of the semiconductor active layer 11, high concentration impurity source and drain regions 11 a and 11 b are formed. Between the source and drain regions 11 a and 11 b, a channel region 11 c is established. Between the source region 11 a and the channel region 11 c and between the drain region 11 b and the channel region 11 c, low concentration impurity LDD regions 16 are formed. On the channel region 11 c, a gate insulating layer 12 and a gate electrode 13 are formed. Over the gate electrode 13, an insulating inter-layer 17 having a plurality of contact holes is formed. The source electrode 18 and the drain electrode 19 respectively contact the source region 11 a and the drain region 11 b through the contact holes formed in the insulating inter-layer 17. On the insulating inter-layer 17, a protective layer 20 is formed, and defines a contact hole through which a pixel electrode 21 contacts the drain electrode 19.

FIGS. 3A-3D show cross-sectional views taken along line III—III of FIG. 1 for illustrating a conventional method of manufacturing the conventional LDD structure of a thin film transistor.

As shown in FIG. 3A, polysilicon is formed on the insulating substrate 10, and patterned to form the semiconductor active layer 11. On the semiconductor active layer 11 and the exposed substrate 10, a first insulating layer and a first conductive layer are sequentially formed. The first conductive layer is a metal material or an impurity doped polysilicon type of conductive material. The first insulating layer and the first conductive layer are patterned according to a prescribed shape to form the gate insulating layer 12 and the gate electrode 13. At the same time, the signal line 13′, connected to the gate electrode 13, and the first storage capacitor electrode 24 are formed (see FIG. 1).

As shown in FIG. 3B, photoresist is deposited (e.g., coated) over the substrate 10, and patterned to form an ion injection or implantation mask 15. Using the mask 15, high concentration impurity ions are injected by ion implantation or doping into the exposed semiconductor active layer 11 to form the source region 11 a and the drain region 11 b. Thereafter, heat treatment or annealing, using a laser, is performed to planarize the semiconductor active layer 11. The mask 15 is completely removed thereafter.

As shown in FIG. 3C, using the gate electrode 13 as a mask, low concentration conductive type ions are implanted into the semiconductor active layer 11 to form the LDD regions 16 adjacent to the source region 11 a and the drain region 11 b. As a result, the semiconductor active layer 11 includes the source region 11 a, LDD region 16, channel region 11 c, LDD region 16, and drain region 11 b.

As shown in FIG. 3D, a second insulating layer, such as oxide material, is formed over the substrate 10 and patterned to form the insulating inter-layer 17. Through the insulating inter-layer 17, contact holes are formed to expose a portion of the source region 11 a and a portion of the drain region 11 b. On the insulating inter-layer 17 and the contact holes, a second conductive material is deposited and patterned to form the source electrode 18 and the drain electrode 19 respectively connected to the source region 11 a and the drain region 11 b through respective contact holes.

Thereafter, a shown in FIG. 2, a third insulating layer is deposited and patterned to form the protective layer 20. Through the protective layer 20, a contact hole is formed to expose a portion of the drain electrode 19. On the protective layer 20, a third conductive material is deposited and patterned to form the pixel electrode 21. The pixel electrode 21 contacts the drain electrode 19 through the contact hole in the protective layer 20.

As described above, forming the conventional LDD structure of a polycrystaline silicon transistor requires the use of photoresist as an ion injection mask to form the LDD regions. That is, over the insulating substrate 10, having the semiconductor active layer 11 and the gate electrode 13, photoresist is coated over the insulating substrate 10 and etched using a mask corresponding to an LDD pattern to be formed later in order to form an ion injection mask. However, use of such a mask results in misalignment and over-etching of the LDD region. As a result, the conventional method of manufacturing an LDD structure of a thin film transistor has problems of accurately and uniformly establishing LDD regions. If the LDD regions are not accurately formed, the thin film transistors will not be uniformly formed, the picture quality uniformity deteriorates, the contrast ratio decreases, and the flicker phenomenon occurs.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improvement in transistor structure which overcomes the above-described and other problems encountered in related and conventional thin film transistors.

Another object of the present invention is to provide a thin film transistor having uniformly sized LDD regions.

To achieve the above and other objects of the present invention, a thin film transistor, comprising: a first insulating layer; an active region formed on the first insulating layer, the active region including a channel region, a source region formed on a first side of the channel region, a drain region formed on a second side of the channel region, a sub-channel region formed between the channel region and at least one of the source region and the drain region, and a first region formed between the channel region and each sub-channel region; a gate insulating layer formed on the channel region and each sub-channel region; a gate electrode formed on the gate insulating layer over the channel region; and a sub-gate electrode formed on the gate insulating layer over each sub-channel region.

These and other objects are further achieved by a method of forming a thin film transistor, comprising: forming an active region on a first insulating layer having a channel region, at least one sub-channel region, and first regions disposed between the channel region and each sub-channel region; sequentially forming a second insulating layer and a first conductive layer over the first insulating layer; patterning the second insulating layer and the first conductive layer to form a gate insulating layer and gate electrode on a channel region of the active layer, and to form a sub-gate insulating layer and associated sub-gate electrode on each sub-channel region of the active layer; forming a mask covering at least a portion of the gate electrode, at least a portion of each sub-gate electrode, and each first region of the active region; implanting impurities into exposed portions of the active region using the mask to form a source region on a first side of the channel region and a drain region on a second side of the channel region such that each sub-gate region is disposed between the channel region and one of the drain region and the source region.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:

FIG. 1 shows a plan view of a conventional liquid crystal display which uses a thin film transistor as a pixel switching circuit;

FIG. 2 shows a cross-sectional view taken along line II—II of FIG. 1;

FIGS. 3A-3D show cross-sectional views taken along line II—II of FIG. 1 for illustrating a conventional method of forming the LCD of FIG. 1;

FIG. 4 shows a plan view of a liquid crystal display which uses a thin film transistor as a pixel switching circuit in accordance with the embodiments of the present invention;

FIG. 5 shows a cross-sectional view taken along line V—V of FIG. 4; and

FIGS. 6A-6D show cross-sectional views taken along line V—V of FIG. 4 for illustrating a method of forming a thin film transistor of the LCD in accordance with the embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the drawings for the purpose of illustrating the preferred embodiments of the present invention, FIG. 4 shows a liquid crystal display having a thin film transistor, and FIG. 5 shows a cross- sectional view taken along line V—V in FIG. 4.

As shown in FIG. 4, a signal line 53′ and a data line 58′ cross each other on an insulating substrate 50 (see FIG. 5). The gate electrode 53 of a thin film transistor extends from the signal line 53′, and the source electrode 58 of the thin film transistor extends from the data line 58′. On both sides of the gate electrode 53, sub-gate electrodes 54 are formed. The sub-gate electrodes 54 are separated from each other by a predetermined distance. Therefore, a plurality of gate electrodes are formed in the present invention.

The sub-gate electrodes 54 are connected to a first storage capacitor electrode 54′ which crosses over the data line 58′. On the first storage capacitor electrode 54′, a pixel electrode 61, made of a transparent conductive material, is formed. The pixel electrode 61 connects with the drain electrode 59. The pixel electrode 61 also functions as a second storage capacitor electrode and together with the first storage capacitor electrode 54′ forms a storage capacitor.

As shown in FIG. 5, a semiconductor active layer 51, made of a polysilicon thin film, is formed on an insulating substrate 50. On both sides of the semiconductor active layer 51, high concentration impurity source region 51 a and drain region 51 b are formed. On an inner side of each of the source region 51 a and the drain region 51 b, sub-channel regions 51 c′ are formed. Between the channel region 51 c and the sub-channel region 51 c′, low concentration impurity LDD regions 56 are formed.

On the channel region 51 c, a gate insulating layer 52 and a gate electrode 53 are formed. On each of the sub-channel regions 51 c′, the gate insulating layer 52 and the sub-gate electrode 54 are formed.

On the gate electrode 53 and the sub-gate electrodes 54, an insulating inter-layer 57 is formed and patterned to define contact holes. Through these contact holes, the source electrode 58 and the drain electrode 59 respectively contact the source region 51 a and the drain region 51 b. And a protective layer 60 is formed over the insulating substrate 50 and patterned to define a contact hole. Through the contact hole formed in the protective layer 60, a pixel electrode 61 contacts the drain electrode 59.

As described above, since the LDD regions 56 are formed between the source and the drain regions 51 a and 51 b, the off current of the drain generated during pixel operation is reduced. This result can be achieved also by forming the LDD regions as offset regions without impurities injected therein.

Furthermore, instead of having a sub-channel region for each of the source and drain regions, a sub-channel region may be formed only on one of the drain and source region sides of the channel region to effectively reduce the leakage current of the drain.

The thin film transistor according to the present invention is, preferably, manufactured according to the method illustrated in FIGS. 6A-6D. FIGS. 6A-6D show cross-sectional views taken along line V—V in FIG. 4 for explaining a method of manufacturing a thin film transistor of a liquid crystal display in accordance with the embodiments of the present invention.

As shown in FIG. 6A, polysilicon is deposited and patterned to form a semiconductor active layer 51 on an insulating substrate 50. On the semiconductor active layer 51 and the exposed substrate 50, a first insulating layer and a first conductive layer are sequentially formed. The first insulating layer is formed using silicon oxide or silicon nitride, and the first conductive layer is formed using a metal material or a conductive material including polysilicon doped with impurities. The first insulating layer and the first conductive layer are patterned according to a predetermined pattern to form a gate insulating layer 52, a gate electrode 53, and sub-gate electrodes 54. At the same time, the signal line 53′, connected to the gate electrode 53, and the first storage capacitor electrode 54′, connected to the sub-gate electrodes 54, are simultaneously patterned and formed (see FIG. 4).

As shown in FIG. 6B, photoresist is coated over the insulating substrate 50, and patterned to form an ion injection or implantation mask 55. The mask 55 completely covers the portions of the semiconductor active layer 51 formed between the gate electrode 53 and the sub-gate electrodes 54, but partially exposes portions of the sub-gate electrodes 54. The position and patterning of the mask 55 is important to insure that high concentration ion implantation is safely and uniformly performed in the portions of the semiconductor active layer 51 except for between the gate electrode 53 and the sub-gate electrodes 54. That is, the mask 55 should be etched so that it partially covers an upper surface of the sub-gate electrodes 54 as shown in FIG. 6B. This provides marginal regions corresponding to a distance ΔL (width of sub-gate electrode) during the photoresist etching process. As a result, misalignment errors and over-etching up to a distance of ΔL can be tolerated without damaging the thin film transistor.

By using the mask 55, high concentration impurity ions are injected into the exposed semiconductor layer 51, which consequently forms the source region 51 a and the drain region 51 b. To smooth out the formed regions, a heat treatment or annealing process using a laser is performed. Thereafter, the mask 55 is completely removed.

As shown in FIG. 6C, using the gate electrode 53 and the sub-gate electrodes 54 as a mask, conductive-type low concentration impurities, similar to the ions injected into the source and drain regions 51 a and 51 b, are injected into the semiconductor active layer 51. As a result, LDD regions 56 are formed between the gate electrode 53 and the sub-gate electrode 54. The semiconductor active layer 51 thus includes, in sequence, the source region 51 a, the sub-channel region 51 c′, the LDD region 56, the channel region 51 c, the LDD region 56, the sub-channel region 51 c′, and the drain region 51 b.

In this example, although the LDD regions 56 made of low concentration impurities are formed in the semiconductor active layer 51, the low concentration impurity ion injection step as described in FIG. 6C can be eliminated so as to use the LDD regions 56 as offset regions. This is advantageous because the offset regions, like the LDD regions 56, reduce the drain's off current.

Additionally, although the high concentration impurity regions 51 a and 51 b were established before the LDD regions 56, it is possible to form the LDD regions 56 first and the high concentration impurity regions thereafter. For example, after carrying out the steps described referring to FIG. 6A, the low concentration ions can be injected into the semiconductor active layer. 51 as shown in FIG. 6C. Then, as shown in FIG. 6B, the ion injection mask 55 is formed to completely cover the gate electrode 53 and partially cover the sub-gate electrodes 54, and high concentration ions are injected using the mask 55. By using this method, the source and drain regions 51 a and 51 b are formed in the semiconductor active layer 51, and the LDD regions 56 are subsequently formed between the gate electrode 53 and sub-gate electrodes 54 in the semiconductor active layer 51. The LDD regions 56 are protected from the high concentration impurity ion injection process and are uniformly established.

As shown in FIG. 6D, a second insulating layer, such as an oxide film, is deposited over the insulating substrate and patterned to form an insulating inter-layer 57. In the insulating inter-layer 57, contact holes are formed to expose portions of the source region 51 a and the drain region 51 b of the semiconductor active layer 51. On the insulating inter-layer 57 and in the contact holes, a second conductive material is deposited and patterned to form the source electrode 58 and the drain electrode 59. The source and drain electrodes 58 and 59 respectively contact the source and drain regions 51 a and 51 b through the contact holes.

As shown in FIG. 5, over the insulating inter-layer 57 and the drain and source electrodes 59 and 58, a third insulating layer is deposited and patterned to form the protective layer 60. Through the protective layer 60, a contact hole is formed through which the pixel electrode 61 contacts the drain electrode 59. Accordingly, the structure of the thin film transistor shown in FIG. 5 is obtained.

According to the embodiments of the present invention, any misalignment of the LDD regions due to inaccurate positioning of the ion injection mask can be reduced. Misalignment errors up to the amount of overlap between of the ion injection mask and the sub-gate electrodes can be tolerated. As a result, transistors having uniformly sized LDD regions can be provided for an active matrix liquid crystal display. The uniformly sized LDD regions allow picture quality uniformity and an increased contrast ratio, and eliminate the occurrence of the flicker phenomenon. Accordingly, the performance of the liquid crystal display is enhanced.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A method of forming a thin film transistor, comprising: forming a semiconductor active layer on a first insulating layer; sequentially forming a second insulating layer and a first conductive layer over the semiconductor active layer; patterning the second insulating layer and the first conductive layer to form a gate insulating layer underlying a gate electrode, and to form a first and a second sub-gate insulating layers underlying a first and a second sub-gate electrodes, respectively; forming an ion injection mask covering at least a portion of each of said first and second sub-gate electrodes, and covering an entire region extending from said first sub-gate electrode to said second sub-gate electrode; implanting a first dose of impurities into exposed portions of the semiconductor active layer using the ion injection mask to form a heavily doped source region abutting a first sub-channel region under said first sub-gate electrode and a heavily doped drain region abutting a second sub-channel region under said second sub-gate electrode; removing said ion injection mask; implanting a second dose of impurities into the semiconductor active layer using the gate electrode and the first and second sub-gate electrodes as a mask to form a first and a second lightly doped regions, wherein said first lightly doped region is between and abutting said first sub-channel region and a channel region underlying said gate electrode, and said second lightly doped region is between and abutting said channel region and said second sub-channel region, and wherein said first and second lightly doped regions are aligned with said channel region and said first and second sub-channel regions in a straight line; and forming a pixel electrode electrically connected to said drain region, wherein a portion of said pixel electrode forms a first electrode of a storage capacitor, and a conductive line connected to said first and second sub-gate electrodes forms a second electrode of said storage capacitor.
 2. The method of claim 1, further comprising the step of: forming a signal line connected to said gate electrode.
 3. The method of claim 2, wherein said conductive line, said gate electrode, said first and second sub-gate electrodes and said signal line are formed simultaneously.
 4. The method of claim 1, further comprising the steps of: forming a third insulating layer covering said gate electrode, said first and second sub-gate electrodes, and exposed portions of said semiconductor active layer and said first insulating layer; forming a first and a second contact holes in said third insulating layer, which expose a portion of the source region and the drain region, respectively; forming a second conductive layer over the third insulating layer; patterning the second conductive layer to form a source electrode in contact with the source region via the first contact hole and a drain electrode in contact with the drain region via the second contact hole; forming a protective layer over the patterned second conductive layer and exposed portions of said third insulating layer; and forming a third contact hole in said protective layer, which exposes the drain electrode, wherein said pixel electrode is formed on a portion of the protective layer and in contact with the drain electrode via the third contact hole.
 5. The method of claim 4, further comprising the step of forming a signal line connected to said gate electrode.
 6. The method of claim 5, wherein said conductive line, said gate electrode, said first and second sub-gate electrodes and said signal line are formed simultaneously.
 7. The method of claim 1, wherein said pixel electrode is formed of a transparent conductive material.
 8. The method of claim 1, wherein said first and second sub-gate insulating layers and said gate insulating layer are vertically aligned with said first and second sub-gate electrodes and said gate electrode, respectively.
 9. The method of claim 1, wherein the first insulating layer is an insulating substrate.
 10. The method of claim 1, wherein the semiconductor active layer is formed of polysilicon.
 11. A method of forming a thin film transistor, comprising the steps of: forming a semiconductor active layer on a first insulating layer; sequentially forming a second insulating layer and a first conductive layer over the semiconductor active layer; patterning the second insulating layer and the first conductive layer to form a gate insulating layer underlying a gate electrode, and to form a first and a second sub-gate insulating layers underlying a first and a second sub-gate electrodes, respectively; implanting a first dose of impurities into the semiconductor active layer using the gate electrode and the first and second sub-gate electrodes as a mask to form a first and a second lightly doped regions, wherein said first lightly doped region is between and abutting a first sub-channel region underlying said first sub-gate electrode and a channel region underlying said gate electrode, and said second lightly doped region is between and abutting said channel region and a second sub-channel region underlying said second sub-gate electrode, and wherein said first and second lightly doped regions are aligned with said channel region and said first and second sub-channel regions in a straight line; forming an ion injection mask covering at least a portion of each of said first and second sub-gate electrodes, and covering an entire region extending from said first sub-gate electrode to said second sub-gate electrode; implanting a second dose of impurities into exposed portions of the semiconductor active layer using the ion injection mask to form a heavily doped source region abutting said first sub-channel region and a heavily doped drain region abutting said second sub-channel; removing said ion injection mask; forming a pixel electrode electrically connected to said drain region, wherein a portion of said pixel electrode forms a first electrode of a storage capacitor, and a conductive line connected to said first and second sub-gate electrodes forms a second electrode of said storage capacitor.
 12. The method of claim 11, further comprising the step of: forming a signal line connected to said gate electrode.
 13. The method of claim 12, wherein said conductive line, said gate electrode, said first and second sub-gate electrodes and said signal line are formed simultaneously.
 14. The method of claim 11, further comprising the steps of: forming a third insulating layer covering said gate electrode, said first and second sub-gate electrodes, and exposed portions of said semiconductor active layer and said first insulating layer; forming a first and a second contact holes in said third insulating layer, which expose a portion of the source region and the drain region, respectively; forming a second conductive layer over the third insulating layer; patterning the second conductive layer to form a source electrode in contact with the source region via the first contact hole and a drain electrode in contact with the drain region via the second contact hole; forming a protective layer over the patterned second conductive layer and exposed portions of said third insulating layer; and forming a third contact hole in said protective layer, which exposes the drain electrode, wherein said pixel electrode is formed on a portion of the protective layer and in contact with the drain electrode via the third contact hole.
 15. The method of claim 14, further comprising the step of forming a signal line connected to said gate electrode.
 16. The method of claim 15, wherein said conductive line, said gate electrode, said first and second sub-gate electrodes and said signal line are formed simultaneously.
 17. The method of claim 11, wherein said pixel electrode is formed of a transparent conductive material.
 18. The method of claim 11, wherein said first and second sub-gate insulating layers and said gate insulating layer are vertically aligned with said first and second sub-gate electrodes and said gate electrode, respectively.
 19. The method of claim 11, wherein the first insulating layer is an insulating substrate.
 20. The method of claim 11, wherein the semiconductor active layer is formed of polysilicon.
 21. A method of forming a switching device, comprising the steps of: forming a semiconductor active layer on a first insulating layer; sequentially forming a second insulating layer and a first conductive layer over the semiconductor active layer; patterning the second insulating layer and the first conductive layer to form a gate insulating layer underlying a gate electrode, and to form a first and a second sub-gate insulating layers underlying a, first and a second sub-gate electrodes, respectively; fanning an ion injection mask covering at least a portion of each of said first and second sub-gate electrodes, and covering an entire region extending from said first sub-gate electrode to said second sub-gate electrode; implanting impurities into exposed portions of the semiconductor active layer using the ion injection mask to form a heavily doped source region abutting a first sub-channel region underlying said first sub-gate electrode and a heavily doped drain region abutting a second sub-channel region underlying said second sub-gate electrode, wherein a first offset region is formed in said semiconductor active layer between and abutting said first sub-channel region and a channel region underlying said gate electrode, and a second offset region is formed in said semiconductor active layer between and abutting said channel region and said second sub-channel region; removing said ion injection mask; and forming a pixel electrode electrically connected to said drain region, wherein a portion of said pixel electrode forms a first electrode of a storage capacitor, and a conductive line connected to said first and second sub-gate electrodes forms a second electrode of said storage capacitor, and wherein said first and second offset regions remain to be non-implanted and are aligned with said channel region and said first and second sub-channel regions in a straight line.
 22. The method of claim 21, further comprising the steps of: forming a third insulating layer covering said gate electrode, said first and second sub-gate electrodes, and exposed portions of said semiconductor active layer and said first insulating layer; forming a first and a second contact holes in said third insulating layer, which expose a portion of the source region and the drain region, respectively; forming a second conductive layer over the third insulating layer; patterning the second conductive layer to form a source electrode in contact with the source region via the first contact hole and a drain electrode in contact with the drain region via the second contact hole; forming a protective layer over the patterned second conductive layer and exposed portions of said third insulating layer; and forming a third contact hole in said protective layer, which exposes the drain electrode, wherein said pixel electrode is formed on a portion of the protective layer and in contact with the drain electrode via the third contact hole.
 23. The method of claim 22, further comprising the step of forming a signal line connected to said gate electrode.
 24. The method of claim 23, wherein said conductive line, said gate electrode, said first and second sub-gate electrodes and said signal line are formed simultaneously.
 25. The method of claim 21, wherein the semiconductor active layer is formed of polysilicon. 